This invention relates to cellular antenna systems and, more particularly, to such systems capable of providing omnidirectional azimuth coverage without coverage reduction due to inter-beam nulling effects, when mounted around the periphery of a large structure.
Increased use of cellular communication systems results in an expanding need for towers or other structures suitable for the mounting of cellular antennas. For many cellular applications the ideal configuration is an antenna system mounted on the top of a tower and arranged to provide omnidirectional azimuth coverage (i.e., substantially uniform coverage 360 degrees around the tower horizontally). However, a combination of factors, including increasing demand and limited supply of suitable towers, plus public objection to new tower locations and proliferation, tends to limit availability and increase the cost of suitable tower top locations for new antenna systems.
Additional antenna mounting locations are available even after the desirable mounting locations at the top of existing towers are occupied. Such locations exist on the sides of large towers and the sides of buildings and other structures. These side locations (e.g., on the side of a large tower) are suitable for many applications not requiring omnidirectional azimuth coverage (e.g., coverage in only a 45 or 90 degree sector).
An attempt can be made to provide omnidirectional coverage from the sides of a large tower. However, with use of four 90 degree beamwidth antennas, for example, the width of the side of a large tower can typically result in the individual antennas being spaced apart laterally by a number of wavelengths at an operating frequency. A result of such arrangement will be that a cellular user located at a distance from the antenna system may be positioned at an azimuth where the lateral edge portions of the beam patterns from two adjacent antennas overlap, so that the user's cellular receiver receives signals from both of the two antennas. Under such circumstances, differences in the path lengths from the two antennas to the cellular receiving antenna may result in signals from one antenna arriving out of phase with signals from the other antenna. The two signals may thus partially or completely cancel each other, so that no usable signal level can be received by the user. The user is thus located in a signal null region of some width and range, which will be typical of a pattern of such null regions at different azimuths around the antenna system. In these null regions the signals from two adjacent widely-spaced antennas cancel each other to a varying degree depending on actual range and azimuth from the transmitting antenna system. The resulting areas of low or no signal reception at various locations thus limit the quality and uniformity of coverage achievable. This nulling characteristic and the resulting limitation on uniform omnidirectional coverage is specifically recognized in ANTENNA ENGINEERING HANDBOOK, R. Johnson, Third Edition, McGraw Hill, 1993. At page 27-18 it is stated:
VHF/UHF base-station antennas are sometimes situated on the bodies of large towers, perhaps up to 10 m (30 ft.) in diameter. It is not economically possible to provide smooth omnidirectional coverage from such a large structure. PA1 suitability for use on the sides of wide towers or other large structures; PA1 improved omnidirectional coverage with antenna-to-antenna lateral spacings of 5, 10, 50 or more wavelengths; PA1 omnidirectional coverage with signal polarization varying with azimuth; PA1 adjacent antennas having cross polarization to prevent null effects; PA1 multibeam capability with low system complexity; PA1 reduced signal processing losses; and PA1 circularly polarized cell antennas for operation with linearly polarized user receiver antennas. PA1 (i) a first set of antennas, each having a beam pattern of a first polarization, and PA1 (ii) a second set of antennas, each at a position between two antennas of said first set and each having a beam pattern of a cross polarization,
This unequivocal conclusion in an antenna handbook reflects the accepted understanding in the prior art that from mounting locations on the sides of a large tower, and using a reasonable number of antennas, smooth omnidirectional coverage was not possible because of signal nulls in overlapping beam areas. Of course, more uniform coverage could be physically achieved by use of a large number of narrow beam antennas, with narrow lateral separation between antenna mounting positions. However, typically it is not an economically feasible solution to use a large number of closely spaced antennas all the way around a large structure.
Systems including interspersed antennas of differing polarization have been described in different configurations for different purposes. See for example U.S. Pat. No. 5,724,666, issued Mar. 3, 1998. However, known prior systems typically do not transmit overlapping same frequency simultaneous beams from widely spaced antennas, and do not describe how to avoid nulling effects which degrade reception from such beams.
Objects of the invention are, therefore, to provide new and improved cellular antenna systems, and such systems providing one or more of the following characteristics and advantages: